This invention relates to a high-speed electromagnetic actuator, and particularly to a electronic valve timing actuator for opening and closing the valves of an internal combustion engine. More particularly, this disclosure relates to an apparatus and method of actuating valves in an internal combustion engine by magnetostrictive transduction.
A conventional method of actuating the valves of an internal combustion engine is by a mechanical camshaft-driven valve train. The camshaft has lobes on it that push against valve lifters or tappets as the camshaft rotates in synchronization with the crankshaft. The linear motion imparted to the lifters by the camshaft is then applied to actuate the valves, either directly or via push rod and rocker arm assemblies, at the appropriate times in the engine cycle.
A second method of actuating the valves of an internal combustion engine is by use of an electromagnetic actuator. This method of actuation uses an armature and stator configuration, wherein the armature may be connected directly or via a hydraulic lifter (i.e., tappet) to the valve stem. Energizing either the opening or closing stator pulls the armature, and therefore the valve, to the respective opened or closed valve position. Because of the large forces involved, it is desirable to provide a xe2x80x9csoft landingxe2x80x9d (i.e., near zero velocity landing) of the armature against the stator core to reduce wear and noise of the valve train. U.S. Pat. No. 5,991,143 to D. Wright and P. Czimmek, teaches a method of providing the above-described electromagnetic actuator with the characteristic of a soft landing using an electronic closed loop control system.
The conventional electronic valve timing actuator consists of an armature, one or more stators, and a solenoid coil for each stator that, when energized, provides the magnetomotive force that generates a magnetic force and causes the armature to move to an open or closed position. The point at which the armature, and therefore the valve, begins to move varies with the spring load in the system, the friction and inertia of the valve, the cylinder pressure, eddy currents, and the ability of the particular design to direct magnetic flux into the working gap. Essentially, the valve will not move until the magnetic force builds at a rate and to a level high enough to overcome the opposing forces and influences noted above. Likewise, until the magnetic force decays to a low enough level for the actuating spring to overcome the valve inertia and magnetic force, the valve will not return to its start position. In a conventional electronic valve timing actuator, once the valve begins opening or closing, it will continue to travel toward the opposite stator position until it impacts the respective end-stop, creating potentially severe wear and noise problems unless a closed loop control, such as described above, is utilized to accomplish a soft landing.
Accordingly, a need exists for an electronic valve timing actuator that can dynamically vary the absolute valve lift and the duration of the valve lift, while providing a soft landing without the need for external control circuitry. The ability to vary the absolute valve lift is absent from the above-described conventional electromagnetic actuator, whose fixed mechanical dimensions prevent dynamically varying the opening valve lift dimension.
One method of achieving controlled trajectory operation is by use of a piezoelectric transducer. Transducers convert energy from one form to another and the act of conversion is referred to as transduction. The piezoelectric effect is a form of transduction. A piezoelectric actuator consists of a stack of piezoceramic or piezocrystal wafers bonded together to form a piezostack transducer. Piezoelectric transducers convert energy in an electric field into a mechanical strain in the piezoelectric material. The piezostack may be attached to the mechanical member, such as a valve lifter or valve stem. When the piezostack has a high voltage potential applied across the wafers, the piezoelectric effect causes the stack to change dimension, thereby actuating the valve. An advantage of piezoelectric actuation is that the piezostack applies full force during the entire armature travel, allowing for controlled trajectory operation.
However, a piezoelectric actuator requires a complex high voltage driver with the capacity to slew hundreds of volts rapidly into a capacitive load while maintaining high voltage isolation. The following derivation explains why it is advantageous to utilize magnetostrictive transduction over electrostrictive transduction in most electronic valve timing applications. Comparing the energy densities in magnetic and electric fields, it can be shown that the energy density in a magnetic field is given by:
B2/2 xcexc, where xcexc=permeability of free space=12.57xc3x9710xe2x88x927 Tm/A;
the energy density in an electric field is given by:
xcex5E2/2, where xcex5=permittivity of free space=8.85xc3x9710xe2x88x9212 C/Nm2; and
thus the ratio of magnetic to electric energy densities=B2/xcexcxcex5E2.
Conservatively, the magnetic energy density is several orders of magnitude greater than the electric energy density, given that most ferromagnetic materials saturate above 1 Tesla (usually around 2 Tesla) and most dielectrics break down at above 100,000 Volts per mm (usually higher).
Accordingly, because magnetic energy density is several orders of magnitude greater than the electric energy density, piezoelectric (i.e., electrostrictive) transduction requires high voltages to generate a useful electric energy density and hence a useful strain in the piezoelectric material, e.g. on the order of approximately 200 volts. Thus, a need exists for an engine valve actuator capable of operating on the magnetic equivalent of the piezoelectric effect, i.e., magnetostriction.
The magnetostrictive electronic valve timing actuator disclosed herein provides the desired soft landing without the need of any external soft landing control circuitry. The term xe2x80x9cmagnetostrictionxe2x80x9d literally means magnetic contraction, but is generally understood to encompass the following similar effects associated with ferromagnetic materials: the Guillemin Effect, which is the tendency of a bent ferromagnetic rod to straighten in a longitudinal magnetic field; the Wiedemann Effect, which is the twisting of a rod carrying an electric current when placed in a magnetic field; the Joule Effect, which is a gradual increasing of length of a ferromagnetic rod when subjected to a gradual increasing longitudinal magnetic field; and the Villari Effect, which is a change of magnetic induction in the presence of a longitudinal magnetic field (Inverse Joule Effect).
The dimensional changes that occur when a ferromagnetic material is placed in a magnetic field are normally considered undesirable effects because of the need for dimensional stability in precision electromagnetic devices. Therefore, manufacturers of ferromagnetic alloys often formulate their alloys to exhibit very low magnetostriction effects. All ferromagnetic materials exhibit magnetic characteristics because of their ability to align magnetic domains. As shown in FIG. 1, strongly magnetostrictive materials characteristically have domains that are longer in the direction of their polarization (North/South) and narrower in a direction perpendicular to their polarization, thus allowing the domains to change the major dimensions of the ferromagnetic material when the domains rotate.
For example, the magnetostrictive alloy Terfenol-D (Tb0.32Dy0.68Fe1.92), is capable of approximately 10 um displacements for every 1 cm of length that is exposed to an approximately 500 Oersted magnetizing field. The general equation for magnetizing force, H, in Ampere-Turns per meter (1 Oersted=79.6 AT/m) is:
H=IN/L; where I=Amperes of current, N=number of turns, and L=path length.
Terfenol-D is often referred to as a xe2x80x9csmart materialxe2x80x9d because of its ability to respond to its environment and exhibit giant magnetostrictive properties. As depicted in FIG. 2, the magnetostrictive response is described by the following equation:
S=dH; where S=strain, d=slope of strain-magnetization curve, and
H=magnetic field intensity.
While the present invention will be described primarily with reference to Terfenol-D as a preferred magnetostrictive material, it will be appreciated by those skilled in the art that other alloys having similar magnetostrictive properties may be substituted and are included within the scope of the present invention.
A magnetostrictive valve actuator is provided. The magnetostrictive actuator has a body having a first cavity and a second cavity, the first cavity having a distal end and a proximal end forming a longitudinal axis. The second cavity forms a hydraulic chamber in communication with the first cavity via a first bore having a first sealing diameter. A first piston is sealably positioned in the first bore, the first piston being displaceable in the direction of the longitudinal axis. A second bore is provided having a second sealing diameter, the second bore being in communication with the hydraulic chamber. A second piston is sealably positioned in the second bore, the second piston being displaceable within the second bore in response to changes in hydraulic pressure within the hydraulic chamber. A magnetostrictive member having a predetermined length is disposed substantially in the direction of the longitudinal axis of the first cavity, the magnetostrictive member being in operative contact with the first piston. A coil for generating a magnetic field is disposed proximate the magnetostrictive element such that magnetic flux passes through the magnetostrictive element upon excitation of the coil, causing the predetermined length to increase, forcing the first piston to be displaced toward the hydraulic chamber and the second piston to move under the influence of the hydraulic pressure created by the displacement of the first piston.
A method of actuating a valve in an internal combustion engine is also provided. The method includes locating a coil for generating a magnetic field proximate a magnetostrictive element having a predetermined length, such that magnetic flux passes through the magnetostrictive element upon excitation of the coil, causing the predetermined length to increase and forcing a first piston to be displaced toward a hydraulic chamber and a second piston to move under the influence of the hydraulic pressure created by the displacement of the first piston.